RESEARCH:

Researchers resolve misunderstanding about how some lithium batteries function

In the push for batteries that store more energy and cost less, many researchers are chasing diminishing performance returns with exotic materials and chemistries, including lithium air, liquid metal and molten salt.

One of the problems is that scientists are still grappling with the fundamental physics behind batteries and are finding out that in some instances, they've been going about it all wrong.

Last week, in the journal Nature Communications, researchers outlined a new understanding of how energy moves within certain types of electrodes in cells, overturning the conventional wisdom that has reigned for more than 80 years.

Until recently, researchers modeled how electrons moved in cathodes and anodes using the Butler-Volmer equation, which describes how electrical currents respond to electrical potentials. Chemist Max Volmer described this relationship in 1930, building on work from chemist John Alfred Valentine Butler based on empirical measurements.

Experiments around the time confirmed these results, but as researchers devised new types of batteries and developed better testing instruments, the model started breaking down.

A glitch 2,000 papers missed?

Peng Bai, a postdoctoral associate at the Massachusetts Institute of Technology and a co-author, said this idea piqued his interest when he came across a Japanese experiment on lithium iron phosphate cells. "The traditional Butler-Volmer equation did not fit [those] data," he said.

It was surprising that scientists didn't fully understand lithium iron phosphate's behavior, given its prevalence. "It's widely used in commercial batteries," Bai said. "This material has been investigated by more than 2,000 papers."

Many past studies assumed that a lithium iron phosphate battery's performance depends on how fast lithium ions can move between the liquid electrolyte and the solid electrode. Bai and his adviser, MIT chemical engineering professor Martin Bazant, tested this with a cell that used a porous electrode with a carbon coating.

Analyzing its performance, the researchers found that the Butler-Volmer equation didn't fit the results well, but another model, the Marcus-Hush-Chidsey theory, matched the energy output. The theory governs how electrons move at the atomic level. In this case, it means that how fast electrons move between the porous electrode and its carbon coating is the main limiting factor in the cell's performance. Lithium ion movement, by contrast, is too fast to play a major role in the battery's performance.

The two models stood apart especially at the edges of the cell's performance. "The difference is really at the high-voltage regime," Bai said. "In my paper, the difference kicks in at voltages larger than 100 millivolts." Here, the voltage is the overpotential, the difference between the observed and the calculated voltage from a cell.

The path to better batteries

Researchers will therefore have to include electron transfer rates in their models for batteries or else real-world performance won't line up with simulations. The findings also open up new paths for optimizing battery performance such as using nanoparticle structures.

Rudolph Marcus, a chemistry professor at the California Institute of Technology who was not involved in this research, described the report as "a big step forward, especially for nanotechnology."

Marcus' work on electron transfer reactions, which earned him the 1992 Nobel Prize in chemistry, formed the basis for the mechanisms outlined in Bai's study. Understanding the fundamentals better could one day unlock major performance gains in batteries, according to Marcus. This would enable a suite of clean technologies from grid batteries to smooth out power variations from wind turbines and solar panels to zero-emissions vehicles.

The approach now is to examine every step in a cell's operation. "When it comes down to details for individual steps, individual processes, there is always room for improvement," Marcus said.